1. Field of the Invention
The present invention relates to a brake device for pressing a friction member against a member to be braked to produce a braking force in which a deformation of a piezoelectric element is utilized to produce the braking force.
2. Description of the Prior Art
Heretofore, a brake device of the above type has been proposed which includes a single piezoelectric element array of stacked piezoelectric elements, each including a piezoelectric material such as piezoelectric ceramic sandwiched between electrode plates, so arranged that when a DC voltage is applied across each of the piezoelectric elements they produce deformations or strains which are added in the direction of the piezoelectric element array to provide a total amount of deformation that is utilized to press a friction member against a member to be braked, thereby generating a braking force (see Japanese LaidOpen Patent Publication No. 60-81527).
In the brake device of this type, the deformation produced by each piezoelectric element is quite small and, consequently, a lot of piezoelectric elements must be stacked in order to obtain a sufficient braking force, with the result that the piezoelectric array is so long that its space factor is poor.
In order to effectively utilize the deformations of a relatively small number of stacked piezoelectric elements, another brake device has been developed (see Japanese Laid-Open Patent Publication No. 60-139563). In this brake device, until a friction member abuts against a member to be braked, the friction member is caused to slide by a motor-drive actuator in a main braking mode. After the friction member has engaged the member to be braked, an auxiliary braking mode employing the piezoelectric element array is carried out to press the friction member against the member to be braked according to the deformation of the piezoelectric element array for generating desired braking forces.
In the above brake device, however, the brake structure includes two different systems for the main and auxiliary braking modes, respectively, and a control system is required to effect switching from the main braking mode to the auxiliary braking mode. As a consequence, the overall arrangement is complex.
Furthermore, in the case where it is desired to use such a brake device to produce anti-lock braking function whereby pumping brake action is produced to prevent an automobile from skidding due to locked wheels at the time of abruptly braking the automobile, it is required to provide a controller to effect on-off control of a voltage applied to one piezoelectric element array, but in such case the braking force varies for a wide range from zero to a normal level, so that the pumping action of the braking force is rough and it is impossible to produce delicate and highly responsive pumping action with reduced width of variation of the braking force.
It might be possible to effect pumping action, with reduced range of variation of the braking force, by varying the level of the applied voltage in a pulse fashion between a lower voltage value and a normal voltage value, rather than by effecting on-off control of the voltage applied across the piezoelectric array as described above. In such case, however, the voltage applied to the piezoelectric element array should be variable, and control means for variable voltage control would be more complicated than the simple onoff control means.